Electron bombardment type ion source for a mass spectrometer



Sept. 8, 1970 v E. B. DELANY ET 3,527,937 ELECTRON BOMBARDMENT TYPE ION souncn FOR A MASS SPECTROMETER Filed April 11. 1967 2 Sheets-Sheet 1 Donald Wlu'iekazd Ralph E. Mayo Sept. 8, 1970 B D'ELANY ETAL 3,527,937

ELECTRON BOMBARDMENT TYPE ION SOURCE FOR A MASS SPECTROMETER Filed April 11.. 1967 2 Sheets-Sheet 2 T h w O z'gf g INVENTORSf 96' Fdz ard B. OeZany 5 4 Wllard 6'. Esiey' Dona 2c? Wiui'ekeaz? Ralph if Mayo United States Patent York Filed Apr. 11, 1967, Ser. No. 630,133 Int. Cl. H01j 39/34 US. Cl. 250-413 12 Claims ABSTRACT OF THE DISCLOSURE An ion source for a mass spectrometer comprising a pair of bar magnet assemblies aligned with a filament and target for confining electron beam passing therebe tween. Each bar magnet assembly comprises a pair of bar magnets separated by soft iron bars with which they are in intimate contact. Small aligned cylindrical channels in the soft iron bars provide means for introducing samples into the ionization source from two directions. Stainless steel shims are provided in the magnet assemblies to reduce variations in magnetic field strength due to temperature cycling as the ion source is heat cycled prior to magnetization.

A rhenium filament in the shape of a hairpin is axially aligned within a cup-shaped grid, preferably a right circular cylinder, which is maintained substantially at the same potential as the filament.

The target comprises a gold plate having a heater mounted closely adjacent thereto on a side away from th filament.

The filament and target are mounted on generally cylindrically shaped ceramic headers, each having an annular groove therein forming an annular skirt providing an annular shadow therearound which does not rapidly become contaminated with stray particles, thus protecting the header from electrical short circuits. The leads supporting the grid are recessed in cylindrical holes in the filament header that provides a similar shadowing function.

RELATED APPLICATION The ion source disclosed and claimed herein is used in a low cost, compact mass spectrometer disclosed and claimed in our co-pending application Ser. No. 630,108, entitled Mass Spectrometer and Means for Supplying a Solid Sample Thereto, filed Apr. 11, 1967. That application is assigned to the same assignee as the present application and is incorporated herein by reference.

BACKGROUND OF THE INVENTION This invention relates to an ion source for a mass spectrometer. More particularly, it relates to an ion source including a magnet assembly internal to the vacuum system of the mass spectrometer providing sample access from two opposite directions and extremely efiicient use of the electrons emitted from the cathode of the ion source.

Ion sources for mass spectrometers comprise a cathode, or filament, and an anode, or target. Electrons are emitted from the cathode and drawn to the anode, which is at a higher positive potential. Because the electrons are not all emitted in the same direction there is a spread in the beam of electrons emitted from the cathode. It is desirable that the electrons that ionize the sample vapor or gas be confined to a small volume near an ion exit slit of the apparatus. According to the prior art, this is accomplished Patented Sept. 8, 1970 ice by providing a magnetic field axially aligned with the electron beam. One method of providing this field, according to the prior art, is with a large horseshoe magnet assembly external to the vacuum chamber containing the ion source. This requires relatively large magnets and magnet structure and also utilizes large air and vacuum gaps in the magnetic circuit. These large gaps lead to the likelihood of stray magnetic fields which, in turn, may lead to undesirable mass separation of the ions provided by the source before they have been accelerated to high velocities.

Another form of magnet assembly and ion source utilized in the prior art provides a pair of bar magnets spaced apart and axially aligned with the cathode and anode. The bar magnets may be quite small and incorporated into the vacuum chamber holding the ion source. However, such assemblies provide sample access to the ion chamber from only one direction; that is, opposite the exit slit of the ion chamber. We tried providing channels through the bar magnets to provide access thereto from the sides. However, this led to stray magnetic field components crosswise between the channels adversely affecting the collimation of the electron beam.

Because the ion chambers are operated over a temperature range in the order of 40 to 300 centigrade, internal magnet assemblies are heat cycled during use. We found that the magnets of these assemblies often became separated from their soft iron pole pieces due to the differences in their coefficients of thermal expansion. This separation causes a change in the reluctance of the magnetic circuit which is not repeatable upon heat cycling.

Prior art cathodes of ion sources for mass spectrometers provide a relatively large initial electron beam spread. Even with magnetic collimation, many of the electrons are lost in passing through the entrance electrostatic lens (a hole in a metal plate) into the ion chamber. Thus, in order to obtain a relatively intense electron beam, prior art ionization sources require high cathode currents. Furthermore, the unused electrons impinging on the ion chamber lead to undesirable local heating effects.

The targets, that is the anodes, of prior art ion sources, often have a dielectric film built up on them formed of cracked hydrocarbon which adversely affects their performance. We have found that heating the target to 350 centigrade greatly reduces the formation of such dielectric films, as does the use of a gold surface for the target.

Another problem of ion sources of prior art mass spectrometers is that the ceramic headers on which the electrodes are mounted become contaminated with conducting films originating in the ion chamber and these lead to early failure of the electrodes through shorting, arcing, and the like. Furthermore, prior art ion sources are hard to service, usually requiring complete breaking of the vacuum of the system and extensive disassembly of the system for repair and replacement of the various elements of the ion source.

SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved ion source for a mass spectrometer.

Another object of the invention is to provide an ion source of the above character providing a Well collimated intense beam of electrons.

A further object of the invention is to provide an ion source of the above character utilizing a small, well-confined magnetic circuit.

A still further object of the invention is to provide an ion source of the above character providing access to the ion chamber thereof from two directions.

A yet further object of the invention is to provide an ion source of the above character not adversely affected by changes in temperature.

Another object of the invention is to provide an ion source of the above character having a long life.

Still another object of the invention is to provide an ion source of the above character not subject to excessive electrical leakage.

A further object of the invention is to provide an ionsource of the above character not easily contaminated by cracked hydrocarbons in the mass spectrometer system.

Still another object of the invention is to provide an ion source of the above character providing convenient maintenance.

A yet further object of the invention is to provide an electrode header for an ion source of the above character.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the features of construction, combinations of elements and arrangements of parts, which will be exemplified in the ion source herein disclosed. The scope of the invention is indicated in the claims.

THE DRAWINGS For a fuller understanding of the nature and the objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of an ion source according to the present invention;

FIG. 2 is a front view, partially cut away and partially in section, of the ion source of FIG. 1;

FIG. 3 is a perspective view of the back plate assembly of the ion source of FIG. 1;

FIG. 4 is a cross-sectional view taken along the line 44 of FIG. 2;

FIG. 5 is a cross-sectional view, taken along the line 55 of FIG. 2;

FIG. 6 is an enlarged cross-sectional view of the cathode header assembly, taken along the line 66 of FIG. 5;

FIG. 7 is an enlarged cross-sectional view of the anode header assembly, taken along the line 77 of FIG. 5; and,

FIG. 8 is a cross-sectional view, taken along the line 88 of FIG. 7.

SPECIFIC DESCRIPTION Now referring to FIG. 2, in general the present invention provides an ion source generally indicated at 10 to which gas samples may be introduced through capillary tube 12 and volatilized solid samples may be introduced through capillary tube sample holder 14 from opposite sides. Tubes 12 and 14 pass through ferromagnetic steel inserts 16 and 18 lying between magnets 20 and 22, and 24 and 26, respectively. The inserts 16 and 18 confine the magnetic field between the magnets, and because they do not contribute to the production of the magnetic field eliminate cross-field components between the assemblies which would occur if the magnetic field potential varied across the channels 28 and 30 through which capillary tubes 12 and 14 pass. Ferromagnetic steel pole pieces 32 and 34 confine the magnetic field at the ends of the ion source 10 so that it is essentially axial between cathode 36 and anode 38 in front of the exit slit 40.

Now referring to FIG. 6, the cathode is mounted to 4 plate 56. The electrons pass through the inner chamber 58 (FIG. 5) and then through a second lens aperture 60 of plate 62 maintained at the same potential as plate 56. They then impinge upon a gold target 64, which is heated by heater 66 to reduce dielectric contamination.

The leads 68 supporting the cup-shaped grid 52 pass through recess channel 70 in the ceramic header 40 so that they will be shadowed from contamination similar to the shadow provided by skirt 44.

Now referring to FIGS. 1 and 2, front plate 72 and back plate 74 have a plurality of holes as at 76 for mounting the ion source 10 within an ionization chamber (as illustrated in our above-identified co-pending application, filed herewith). The mounting may be by means of ceramic spacers or glass spheres, as is preferred. The ends of the plates 72 and 74 are cut out to permit entrance of the sample injecting probes. The left sides of the plates are provided with metal straps 78 and '80 for holding capillary tube 12, which may be retracted into the larger tube 82 when it is desired to remove the ion source 10 from the ionization chamber.

The magnets 20 through 26 and cold rolled steel inserts 16 and 18 are spot welded to straps 84 and 86 as are pole pieces 32 and 34. Four stainless steel shims, 88, 90, 92 and 94, each two-thousandths of an inch thick, are provided between the poles of the magnets 20 through 26 and the pole pieces 32 and 34. An inherent low reluctance gap exists at these interfaces and at the interfaces between the magnets 20 through 26 and the inserts 16 and 18. These effective air gaps are very thin, in the order of one-thousandth of an inch. When the ion source 10 is heated, they change in thickness, changing the effective magnetic field within the inner chamber 58 or causing an asymmetrical field which spoils the collimation of the electron beam. By providing a total controlled fourthousandths of an inch gap on each of the magnet assemblies, minor changes during heat cycling have less effect on the magnetic field configuration. Alternatively, the entire assembly may be beat cycled from room temperature to 300 centigrade for three cycles to at least eliminate any mechanical hysteresis in these effective interface gaps and the magnets are then magnetized.

The headers 40 and 96 set within cylindrical holes in the ferromagnetic steel pole pieces 32 and 34 and are held therein by means of screws 98 and 100 respectively. The electrostatic lens plates 56 and 62 are welded to straps 102 and 104 which, in turn, are welded to back plate 74, as shown in FIG. 3.

The magnets 20 through 26 are formed of alnico and all other metal parts are preferably stainless steel.

Referring to FIG. 6, in an alternative cathode structure, the grid 52 is formed of six mil platinum wire coiled for six turns into a frustum of a cone, the inner diameter of the small end of the gold-brazed wire being 0.030 inch and the inner diameter of the large end being 0.075 inch. In this configuration the grid is held at a negative potential with respect to the filament 46 to provide a greater repelling action.

The grid 52 is mounted to leads 68 which are spot welded to the inside of grid 52, as shown in FIG. 6.

The lens plate 56 is maintained at 5 to 100 volts potential positive with respect to the filament 46 and the magnetic field in front of the exit slit 40 is to 200 gauss. We have found that ninety percent of the electrons emitted from the cathode 36 arrive at the anode 38 if the following cathode dimensions in inches are used, as shown in FIG. 6:

a=0.210 g=0.060 b=0.015 h=0.140 0:0.022 i=0.180 d=0.l20 j=0.070 e=0.125 k=0.030 f=0.040

The most critical dimension appears to be the ratio of j to k which must be kept within approximately plus or minus 5% of the ratio given to properly collimate the electron beam.

New referring to FIG. 7, the anode plate 38 is mounted to a central lead 106 passing through header 96. Plate 38 is perpendicular to the electron beam and preferably has a gold surface. It may either be solid gold or gold-nickel laminate. The gold surface appears to be less subject todielectric contamination than other surfaces. To further reduce dielectric contamination, a heater 108 is provided mounted as shown in FIG. 8 and connected to leads 110 and 112 for electrical supply. The plate 38 is preferably maintained at a temperature of 250 to 300 centigrade.

Anode header 96 is provided with an annular shadowing groove 114 for the same purpose as groove 42 of cathode header 40.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention, which, as a matter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. An ion source for a mass spectrometer comprising:

means providing an ionization chamber;

means for introducing a sample material to said chamber;

an anode disposed at one end of said ionization chamber;

a cathode disposed at an opposite end of said ionization chamber and adapted for emitting electrons for bombarding sample molecules in said chamber to form ions therefrom; and,

a pair of bar magnet assemblies arranged for collimating the electron beam, including:

(a) a pair of bar magnets, and

(b) a body of ferromagnetic material interposed between said bar magnets and in contact therewith, said body including a passage extending therethrough for providing access to said ionization chamber.

2. An ion source for a mass spectrometer as defined in claim 1, further defined in that said passage is wholly surrounded by said ferromagnetic material.

3. An ion source for a mass spectrometer as defined in claim 1 wherein each of said magnet assemblies are identical.

4. An ion source for a mass spectrometer as defined in claim 1, and

a pair of pole pieces engaged with and linking the ends of said magnet assemblies; and

at least a'pair of shims of high reluctance material,

each interposed between one of said magnet assemblies and one of said pole pieces.

5. An ion source for a mass spectrometer as defined in claim 4, further defined in that said pole pieces include apertures therein; and:

ceramic headers (a) inserted in said apertures (b) said ceramic headers each having a generally annular skirt thereabout forming an annular shadow as illuminated from the interior space between said magnet assemblies.

6. And ion source for a mass spectrometer as defined in claim 5, and:

an electron source (a) mounted to one of said headers comprising:

( l) a small cathode, and (2) a generally cup-shaped metal grid axially aligned therewith at a fixed potential with respect thereto.

7. An ion source for a mass spectrometer comprising:

means providing an ionization chamber;

means for introducing a sample to said ionization chamber;

an anode electrode disposed at one end of said ionization chamber;

a small cathode electrode disposed at an opposite end of said ionization chamber and adapted for emitting electrons for bombarding sample molecules in said chamber to form ions therefrom;

a generally cup-shaped metal grid electrode axially aligned with respect to said cathode electrode, maintained at a fixed electric potential with respect to said cathode electrode, said cathode and said grid electrodes positioned within a second chamber of said ion source which is maintained at an equipotential, and,

means for establishing a magnetic field for collimating the electron beam.

8. An ion source as defined in claim 7 wherein said grid is substantially in the form of a right circular cylinder and substantially at the potential of said cathode.

9. An ion source for a mass spectrometer as defined in claim 7 wherein said anode is, and:

(a) aligned with said cathode, and

(b) formed as a flat plate positioned perpendicular to the axis of said cup and a heater closely adjacent to said anode on the side thereof away from said cathode.

10. In an ion source for a mass spectrometer, a header for an electrode thereof, comprising: a generally cylindrical ceramic structure having a segment thereof positioned within a chamber of the ion source and subject to disposition thereon of electrically conductive particles, said segment including an annular skirt forming a continuous annular shadow thereabout when illuminated from the direction of said electrode.

11. The header defined in claim 10 for supporting two metal elements on leads requiring electrical isolation, further defined in that the leads to one of said elements enter said header through recessed passages whereby the recessed portions of said leads are shadowed from surface contamination.

12. In an ion source for a mass spectrometer having a header for leads requiring electrical isolation and including a segment thereof extending into a chamber of UNITED STATES PATENTS 2,838,676 6/ 1958 Raible et al. 2,911,531 11/ 1959 Rickard et al. 2,958,775 11/ 1960 Robbins et al. 3,217,160 11/1965 Craig et al.

ARCHIE R. BORCHELT, Primary Examiner A. L. BIRCH, Assistant Examiner US. Cl. X.R. 313-231 

